Metal loss inhibitors and processes therewith

Details Metal loss inhibitors and processes therewith Metal loss inhibitors and processes therewith

1999-09-20

2002-02-05

Carrillo, Sharidan (Department: 1746)

Cleaning and liquid contact with solids

Processes

For metallic, siliceous, or calcareous basework, including...

C134S003000, C134S022160, C134S022170, C134S022190, C134S026000, C134S030000, C134S036000, C134S041000, C510S247000, C510S249000, C510S253000, C510S263000, C510S421000, C510S487000

Reexamination Certificate

active

06344090

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the cleaning of ferriferous metal surfaces with alkaline aqueous solutions to remove scale and like materials from the surfaces. The cleaning solutions in question contain organic molecules that contain at least two —OH moieties (which may or may not be part of carboxyl or phosphonyl moieties) positioned within the molecule in such a way that the two oxygen atoms are separated from each other by at least two carbon atoms and therefore can readily form sterically unstrained ring structures with at least five atoms per ring, by chelating coordinate bonding to an iron cation.
2. Statement of Related Art
The word “scale” when used herein includes any solid deposit formed on a solid ferriferous metal surface as a result of contact between the metal surface and an aqueous solution in liquid or vapor state. The formation of such scale is a notorious problem in boiler and heat exchanger tubes through which large volumes of aqueous solutions flow and are subjected to temperature variations. These temperature variations can cause solutes in the aqueous solution to become less soluble, can evaporate some of the water in the aqueous solution, and/or can promote chemical reactions within the aqueous solution. All of these possible actions can convert some of the solute in an aqueous solution at one temperature to change to a solid phase at a different temperature and thereby cause deposits of scale. Metal surfaces which these aqueous solutions contact are often a major source of the temperature variations to which the aqueous solutions are subjected, and scale frequently forms on the metal surfaces as a result. Even a thin layer of scale can substantially impede heat transfer from a metal surface to a liquid in contact with it, and if left untreated, scale will eventually completely plug at least some of the passageways through which the scale forming aqueous solution(s) previously flowed.
Accordingly, it is well established practice to clean scale from metal surfaces in industrial process equipment through which large volumes of dilute aqueous solutions, economically valued primarily for their water content, regularly pass. This is particularly true for the very large volume electric power industry, which makes extensive use of cooling water in such equipment during the generation of electricity from either fossil fuel or nuclear reaction.
It is strongly preferred to use a cleaner that dissolves the scale to be removed, rather than merely dislodging it and relying on mechanical motion to remove the still solid but now suspended particles that formerly constituted the scale from what are often narrow passageways. For this and other reasons, the most preferred cleaning solutions for cleaning scale from heat exchanger and/or boiler surfaces are aqueous solutions that include chelating molecules as described above. The cleaning solutions, usually have a pH value between about 6 and 10. The molecules that chelate iron cations are usually also capable of chelating the metal ions in some of the most common constituents of scale, particularly calcium and magnesium sulfates and carbonates. In part because of the presence of these chelating constituents, the commonly used cleaning solutions, unless inhibited, are often capable of unwanted dissolution of underlying ferriferous metal along with their desired dissolution of the scale adhering to the metal.
Advantageous inhibitors reduce the rate of dissolution of clean metal as much as possible while reducing the rate of dissolution of surface scale and like contaminating materials as little as possible. One common quantitative method of rating inhibitors used in the art, and in this description below, involves measuring the corrosion rate of the metal to be cleaned in a solution containing the type and concentration of other solutes to be used for cleaning, measuring the corrosion rate in a solution otherwise the same except for adding the inhibitor to be used, and then reporting the results as the percent “protection” of the inhibitor. The percent protection is defined by the following equation: P=100[1-(R
i
/R
u
)], where P=percent protection, R
i
=corrosion rate of the metal in the inhibited solution, and R
u
=the corrosion rate of the metal in the uninhibited solution containing the same constituents, except for inhibitor, as the inhibited solution. (The corrosion rates for R
u
and R
i
should of course be in the same units when this calculation is made.)
A variety of inhibitors for neutral to alkaline chelating cleaning solutions are known in the art, but none have been found completely satisfactory for all applications.
DESCRIPTION OF THE INVENTION
Objects of the Invention
A major object of this invention is to provide inhibitors and inhibited neutral to alkaline chelating aqueous solutions that provide greater percent protection values to at least one type of ferriferous metal than is provided by any prior art inhibitor. Other concurrent or alternative objects are: to provide effective inhibition at low cost, particularly by utilizing combinations of ingredients that are more effective than any single ingredient in the combination would be alone, or in other words, are synergistic; and to avoid carcinogens and/or otherwise toxic ingredients. The desired freedom from carcinogens includes freedom from carcinogenic products formed by reaction among the components of the inhibitor within 1000 hours of mixing, as well as freedom from known carcinogens directly added to the inhibitor mixtures. Other objects will appear from the description below.
General Principles of Description
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the objects of the invention; and the terms “solution”, “soluble”, and the like are to be understood as including not only true equilibrium solutions but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 1000 hours.
SUMMARY OF THE INVENTION
It has been found that particularly effective inhibition of chelating aqueous neutral to alkaline solutions can be achieved by use of an inhibitor that comprises, preferably consists essentially of, or more preferably consists of the following components:
(A) an amount of a component selected from the group consisting of secondary and tertiary amine molec

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